In recent years, a series of installation inspections has shown that FC switches currently fall short of the requirements necessarily placed on the dependability of the cutout in a circuit breaker in the event of a fault. (Biegelmeier, G. and Kieback, Dr. The Problem of Dependability in Fault Current Circuitry, Bulletin of the International Section of the IVSS for Prevention of Accidents in the Workplace and Occupational Diseases through Electricity, issue 11, 1990).
Nowadays an average failure of several per cent must be reckoned with when examining the operability of FC switches with an installation life of up to ten years, whereby closer breakdown reveals that with an installation life of ten years or more even 10% of the switches do not work. Since there is a growing tendency for FC switches to be installed in millions of units per year, there is a problem to be addressed.
Cause of failure of FC switches lies in the construction principle used in FC switches currently on the market. They use either highly sensitive permanent magnetic triggers in order to save on material for the summary current transformer, as in EP-A-228 345, 351 674 and 293 702, or use electronic circuits with numerous components which are constantly connected to mains voltage and thus have only a limited life, as in EP-A-252 693, 152 043. They are not only exposed to excess voltage, but also consume electrical energy, since the power dissipated by this type of FC component is around one Watt (Solleder. R. Why Fault Current Circuit Breakers with voltage-independent Triggering? vol. 107 (1986), No. 20, pp. 938-945).
Voltage-independent power storage circuits can certainly actuate less sensitive permanent magnetic triggers, but require high-guard and interference-prone switch locks. This solution has been known for quite some time (AT-PS 197 468). It is described in modified form in DE-B-25 40 81 5 and CH-A-656 262.
The construction principle, in which permanent magnetic triggers are used for triggering the FC switches, leads to a dead-end when following the modern tendency of developing more and more highly sensitive triggers. The polished surfaces between trigger armature and yoke must be machined more and more precisely and have a tendency to exhibit a sticking phenomena, the causes of which are not yet clear and which might be found in microcrystalline solid mass formation. Highly developed cleaning processes in extremely clean rooms do improve dependability, but provide no real cure.
There is also the possibility of using an operating current trigger in place of a permanent magnetic trigger.
This type of trigger has been used with success for quite some time for electromagnetic quick breaking of automatic cutouts (AC). Even with the use of storage switches, the power of the summary current transformer is insufficient to actuate an operating current trigger. For this reason, voltage-independent electronic amplifying circuits were used hitherto, whose input is connected to the secondary winding of the summary current transformer and which actuate the operating current trigger mainly by means of thyristor circuits during flow of fault currents of corresponding strengths (see AT-B-378 444). As a result of electronic components in constant contact with the mains supply, this solution has the disadvantages mentioned above with respect to the dependability of the triggering after long installation periods.